Mercury cathode electrolysis



Jan. 7, 19 69 W.J. FRIEMEL ETAL 3,420,757

MERCURY CATHODE ELECTROLYSIS Filed Feb. '21. 1966 PURIFIED WATER SOURCES NaOH SOLUTION PURIFIED WATER LOW PRESSURE ST EAM COLLECTION TANK FIG. I

PRIOR ART- PURIFIED WATER LOW PRESSURE STEAM CONDENSER w w R & F H T K HECI'FTCN RAU R LT UWO U L P S P w ATTORNEY United States Patent 6 Claims This invention relates to an improvement in the process of obtaining alkali metal hydroxide and chlorine from an alkali metal chloride brine, by electrolysis of the brine in a flowing mercury cathode electrolytic cell, and more particularly relates to an improvement in the water circulation system to the amalgam decomposer and regenerated mercury washing tank whereby losses of mercury and alkali metal hydroxide from the system may be minimized.

The process for obtaining alkali metal hydroxide and chlorine from alkali metal chloride brine, particularly from sodium chloride and potassium chloride brines, by electrolysis of the brine in a flowing mercury cathode electrolytic cell has been widely practiced commercially for many years. In this process an alkali metal chloride brine containing the solute in an amount very near the equivalent of a saturated aqueous solution, is passed into a flowing mercury cathode cell having the anodes, usually of graphite, positioned close to the flowing stream of mercury, and an electrolytic current is passed between the anodes and cathode. The products of the electrolysis are chlorine which is evolved at the anode and escapes from the cell through a suitably positioned conduit opening into the cell above the brine level, and the alkali metal, particularly sodium or potassium, is deposited on and dissolves in the flowing stream of mercury to the extent of about 0.1%, and forms an amalgam therewith.

In order to recover the alkali metal hydroxide, such as sodium hydroxide or potassium hydroxide, the alkali metal amalgam is introduced into a decomposer wherein the amalgam, in contact with an electrical conductor such as pieces of graphite, flows counter-currently to a stream of water, the alkali metal of the amalgam reacting with the water to form the alkali metal hydroxide which is recovered from the decomposer. The regenerated mercury from the decomposer, containing about 0.01% of residual alkali metal, flows from the decomposer into a pumptank, which is merely a sump and quenching vessel, where the regenerated mercury is contacted with water to reduce the temperature, as well as further to reduce the amount of residual alkali metal, such as sodium or potassium, remaining in the regenerated mercury. From the pump-tank, the regenerated mercury is recycled to the electrolytic cell as the flowing mercury cathode in which further quantities of the alkali metal are deposited and dissolved, in the electrolysis operation. Further details of the electr0lysis process and the appartus involved may be had by reference to Kirk-Othmer, Encyclopedia of Technology, 2nd edition (1963), vol. I, pp. 688695, inclusive.

It is the water circulation system including water introduced into the decomposer and into the pump tank to which the present invention is directed and, in order that those skilled in the art may better understand the principles of the present invention in view of the ordinary practices in the prior art, reference may be had to the accompanying drawings wherein:

FIGURE I is a diagrammatic flow sheet of the ordinary mercury cell process for the recovery of chlorine and caustic soda or caustic potash, in a flowing mercury cathode electrolysis cell, including the prior water circulation system from the inlet end box through the pump tank; and

FIGURE 11 is a diagrammatic fiow sheet of the improvement in the water supply circulation system to the amalgam decomposer and the pump tank to which the present invention is directed.

Referring now to FIGU RE I, in the customary arrangement there are the flowing mercury cathode cell 2 and the amalgam decomposer 4 and pump tank 6, the mercury cathode cell 2 having anodes 10 appropriately supported from the roof of the cell and being positioned a short distance from the flowing mercury film which acts as the cathode in the cell. The alkali metal amalgam formed in the electrolytic process flows therefrom through amalgam outlet end box 16 and amalgam line 8 into the amalgam decomposer 4 in countercurrent contact with a stream of purified water, such as dimineralized Water, the .water reacting with the amalgam in the decomposer. In order for the water to react with the amalgam, the decomposer must contain an electro-conductive material such as piems of graphite which serve two functions, i.e., to increase the surface of the amalgam in contact with the water, and to act as conductor of the electrons for the reaction of the amalgam with water in forming the alkali metal hydroxide solution, such as caustic soda or caustic potash, which issues from the decomposer. Hydrogen is also evolved by reaction of the alkali metal in the amalgam with the water, and is vented from the decomposer by any suitable means for its disposal or recovery. The decomposer, as shown on FIGURE 1, may also have an external heat exchanger or steam jacket, for maintaining the desired temperature within the decomposer during the decomposition of the amalgam, i.e., temperatures approaching the boiling point of water.

Since it is desirable that the mercury which is introduced into the electrolysis cell be as free as possible from impurities and occluded materials, and since it is also desirable to have the solution of caustic soda or caustic potash issuing from the decomposer in as pure a state as possible, purified water is used to wash the mercury prior to its entrance into the electrolysis cell, to decompose the amalgam in the decomposer, and to wash the regenerated mercury issuing from the decomposer, in the pump tank 6. Water from various sources may be used for this purpose, such as demineralized water from a bed of ion exchange resin, or steam condensate from heat exchangers used in other parts of the system, all of such water from various sources being collected in a purified water collection tank as shown.

From FIGURE I it .will be noted that in the prior art system purified water from the purified water collection tan-k is introduced into the decomposer 4 and issues therefrom as an aqueous solution of caustic soda or caustic potash, and a second stream of purified water is introduced into the mercury inlet end box 12 of the electrolysis cell where it washes the mercury prior to the mercury being introduced into the electrolysis zone of the cell. The efiluent water from the inlet end box 12 is then conducted to the pump-tank 6 where it is brought into contact with regenerated mercury from the decomposer, the regenerated mercury stream being introduced into the pump tank 6 generally below the level of the water introduced from the inlet end box 12. The water introduced into the pump tank 6 has two functions, first to quench the stream of mercury to reduce its temperature prior to the mercury being pumped back into the electrolysis cell, and second to wash the mercury to remove some of the residual alkali metal remaining amalgamated in the regenerating mercury, and thereafter is passed to a mercury trap 14 in which entrained mercury particles remaining in the wash water are settled out, and the effluent from the mercury trap is then passed to the sewer. Water which is passed into the amalgam outlet end-box to wash the amalgam prior to its introduction into the decomposer 4 is generally directed through the same route as that from the mercury inlet end-box, although this stream may be used in brine treatment because of its slight alkalinity in accordance with prior art practices.

It will be apparent that the above-described system is suitable for purging impurities with the purified wash water streams used in washing the regenerated mercury in two separate stages, but it will also be appreciated that the system described above is vulnerable as regards the loss of mercury from the system to the sewer, in the event that the mercury trap 14 fails to any appreciable degree in its function as a means of removing entrained mercury from the efliuent stream of water from the pump tank 10. Also, and previously though to be of somewhat lesser importance however, there is the loss of alkali metal hydroxide, such as caustic soda or caustic potash, in the water stream which ultimately finds its way to the sewer from the pump tank 6.

It has now been found that by isolating the water streams used in washing the mercury inlet end-box 12 and the amalgam outlet endabox 16 from the water stream used in washing the regenerated cercury in pump-tank 6, and combining this latter stream with the water stream introduced into the decomposer 4, the economies realized from material recovery are appreciable and considerably more than offset the slight increase in manufacturing costs generated by added equipment, repairs, labor, and depreciation allowances, so that a highly favorable return on investment is realized.

Referring now to FIGURE 11 of the drawings, for a somewhat more detailed description of the process of the present invention, in which figure the same numerals as those used in FIGURE I are used to identify corresponding apparatus and parts thereof for clarity.

In the process diagrammatically illustrated in FIG- URE II, and in marked contrast to the process shown in FIGURE I and described in connection there-with, water from the purified water collection tank, which was previously noted, may be demineralized water from an ion exchange resin bed, or may be condensate from a heat exchanger such as the steam jacket associated with the decomposer 4, is pumped into the decomposer 4 near the bottom thereof and flows upwardly through the decomposer forming a caustic-alkali solution, such as caustic soda or caustic potash, with the corresponding regeneration of the mercury. The regenerated mercury is then fed into the pump-tank 6, wherein it is washed with a stream of water from the purified water collection tank to remove, insofar as possible, any remaining alkali metal dissolved in the mercury, and at the same time to quench or cool the mercury stream from the decomposer 4, after which the effluent, or overflow, water containing alkali metal hydroxide from the washing step in the pumptank 6 is pumped into the purified water collection tank and combined with water entering from the various sources of purified water. The volume of water fed to the decomposer 4, from the purified water collection tank at the start-up of operation of amalgam decomposer 4 is desirably somewhat greater than the volume of water fed to the pump-tank 6 from the purified water collection tank, and it will be appreciated that there may be a gradual increase in the alkali metal hydroxide concentration in the purified water collection tank. But since it generally is desirable that the alkali metal hydroxide be removed from the decomposer 4 at a substantially constant concentration, the feed rate to the decomposer from the purified water collection tank may be increased to compensate for this rising alkali metal hydroxide concentration, until the system is at equilibrium, balance out by the amount of alkali metal hydroxide picked up at the pump-tank 6, the amount of purified water supplied to the purified water collection tank from the various available sources, and thus by the amount of alkali metal hydroxide in the water fed to the decomposer 4.

It may be desirable, depending upon specific conditions in a given installation, to withdraw a side stream of the efliuent or overflow water from the pump-tank and to pass this side stream through a heat-exchanger where it is cooled to somewhat above room temperature before returning to the purified water collection tank. Such an arrangement would be desirable for temperature control in a system where the temperature of the pump-tank effluent water is likely to rise over to 200 F., due to seasonal changes in the temperature of the ambient atmosphere.

It may also be desirable in a specific system to provide traps for the larger suspended particles of mercury in either the side stream, or the main stream, or both, of the effluent or overflow from the pump-tank 6, and the installation or not of such traps does not, of course, alter the manner in which the principles of the present invention are applied.

EXAMPLE A system essentially the same as that shown in FIGURE II, and described in connection therewith, for a flowing mercury cathode cell plant producing chlorine and sodium hydroxide from a sodium chloride brine, except that a portion of the quench water overflow from the mercury pump tank is passed through a heat exchanger and a mercury recovery trap before being returned to the water collection tank, is operated in conjunction with 51 amalgam decomposer units. These operations are performed to minimize mercury accumulation in the water collection tank and to ensure that temperature increases in the water system are not so great as to promote boiling in the mercury pump-tank washing and quenching operation.

Table 1 reflects the water flow rates and flow line temperatures encountered in operation.

Temperature after heat exchanger.

While the holding time in the mercury pump-tank during washing and quenching permits cooling of the pump tank water to occur by radiation, and there are equivalent heat losses by radiation and convection in the water collection tank, no boiling is experienced at the mercury pump-tanks although the feed water to these tanks is 5 F. warmer than that employed when using the conventional water circulation system of FIGURE I. Appreciable mercury and sodium hydroxide savings are realized for the 51 amalgam decomposer units attached to the central water collection tank in that an average of 2.75 tons per day of sodium hydroxide are recovered, which would have been discharged from the process under the conventional water system and the additional mercury retained in the system and not lost to waste water to the sewer, is readily set at 1,000 to 1,500 pounds per year as determined by laboratory analysis of efiiuent streams sent to sewer.

Without intending to be limited by the above example, and recognizing that minor alterations in the water recycle system are within the skill of the art,

We claim:

1. In the electrolytic process for producing alkali metal hydroxide and chlorine from alkali metal chloride brine including the steps of electrolytically decomposing the alkali metal chloride brine in an electrolytic cell having anodes and a flowing mercury cathode to release chlorine at the anodes and form a dilute alkali metal amalgam,

removing said amalgam from said cell, passing said amalgam in contact with water and "an electrically conductive material in an amalgam decomposer to form alkali metal hydroxide and hydrogen and to regenerate said mercury, washing said regenerated mercury with a stream of water in a washing zone outside said decomposer, and returning the washed regenerated mercury to said electrolytic cell as the flowing mercury cathode,

the improvement which includes passing a stream of purified water to a collection zone, passing a first portion of water from said collection zone to said amalgam decomposer, and passing a second portion of water from said collection zone into contact with regenerated mercury from said decomposer in said washing zone, and returning the efiluent water from said washing zone to said water collection zone.

2. The method of claim 1, in which the alkali metal chloride brine is selected from the group consisting of sodium and potassium chloride brines.

3. The method of claim 1 in which the improvement includes removing a side stream of the eflluent water from said washing zone, cooling said side stream of eflluent water, and returning said cooled side stream of effluent water to said water collection zone.

4. The method of claim 3, in which the alkali metal chloride brine is selected from the group consisting of sodium and potassium chloride brines.

5. The method of claim 1, in which the improvement includes regulating the flow of said portion of water from said collection zone to said decomposer to correspond to changes in the alkali metal hydroxide concentration in said water collection zone, whereby the efliuent alkali metal hydroxide solution from said decomposer may be maintained at relatively constant concentration.

6. The method of claim 5, in which the alkali metal chloride brine is selected from the group consisting of sodium and potassium chloride brine.

References Cited UNITED STATES PATENTS 2,949,412 8/1960 Neipert et al. 204-99 3,213,006 10/1965 Crain et a1 204-99 3,329,595 7/1967 Barbato et al. 20499 JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

U. s. 01. XtR. 

1. IN THE ELECTROLYTIC PROCESS FOR PRODUCING ALKALI METAL HYDROXIDE AND CHLORINE FROM ALKALI METAL CHLORIDE BRINE INCLUDING THE STEPS OF ELECTROLYTICALLY DECOMPOSING THE ALKALI METAL CHLORIDE BRINE IN AN ELECTROLYTIC CELL HAVING ANODES AND A FLOWING MERCURY CATHODE TO RELEASE CHLORINE AT THE ANODES AND FORM A DILUTE ALKALI METAL AMALGAM, REMOVING SAID AMALGAM FROM SAID CELL, PASSING SAID AMALGAM IN CONTACT WITH WATER AND AN ELECTRICALLY CONDUCTIVE MATERIAL IN AN AMALGAM DECOMPOSER TO FORM ALKALI METAL HYDROXIDE AND HYDROGEN AND TO REGENERATE SAID MERCURY, WASHING SAID REGENERATED MERCURY WITH A STREAM OF WATER IN A WASHING ZONE OUTSIDE SAID DECOMPOSER, AND RETURNING THE WASHED REGENERATED MERCURY TO SAID ELECTROLYTIC CELL AS THE FLOWING MERCURY CATHODE, THE IMPROVEMENT WHICH INCLUDES PASSING A STREAM OF PURIFIED WATER TO A COLLECTION ZONE, PASSING A FIRST PORTION OF WATER FROM SAID COLLECTION ZONE TO SAID AMALGAM DECOMPOSER, AND PASSING A SECOND PORTION OF WATER FROM SAID COLLECTION ZONE INTO CONTACT WITH REGENERATED MERCURY FROM SAID DECOMPOSER IN SAID WASHING ZONE, AND RETURNING THE EFFLUENT WATER FROM SAID WASHING ZONE TO SAID WATER COLLECTION ZONE. 